Traditionally, the pistons adapted for use in an internal combustion engine have been of unitary construction made from cast iron or forged steel. As fuel economy has become a greater factor in engine design, attempts have been made to reduce engine weight. Some of these efforts have focused on redesigning the piston to reduce or eliminate portions which are unnecessary to the operational or structural requirements of the piston.
One early suggestion for reducing piston weight is disclosed in U.S. Pat. No. 2,964,364 to Morgan which discloses a design for a single piece piston. This design eliminates unnecessary structure from the pillar supports between which the connecting rod is pivotably connected to the piston by a piston pin. However, weight reduction by merely reducing the structural size of the piston is limited by practical concerns for maintaining reliability and adequate strength of the pistons.
For a variety of reasons, including the need to achieve further size and weight reduction, two part piston assemblies have been developed. One example of a reduced weight, two part piston is discussed in Ayoul, U.S. Pat. No. 4,662,319 wherein a piston assembly includes a high strength piston head formed of cast iron. The piston head is joined to a guiding skirt formed of lighter weight material.
Another approach to reducing weight by replacing the more conventional high density piston material with lighter metals such as high strength aluminum alloys is disclosed in U.S. Pat. No. 3,971,355 to Kottmann. Kottmann '355 discloses a piston pin assembly including a relatively short hollow truncated ovoid rod 6 and bolt 11 which connect piston head 2 to piston rod 7. Hollow rod 6 extends only between support members 4 and 5 which integrally depend from piston head 2. Bolt 11 extends through the hollow portion of rod 6 and beyond support members 4 and 5 to engage shank part 19 separate from piston head 2. Bolt 11 is narrow at its central portion where it connects piston head 2 to piston rod 7 and where it receives the combustion gas pressure exerted on piston head 2; yet bolt 11 is wider at its ends where it connects shank part 19 to the remainder of piston 1. Shank part 19 is rotatably connected to piston 1 via bolt 11 to form a pendulating piston assembly wherein shank part 19 guides the piston in the cylinder without being influenced by piston head 2.
While useful for the purposes disclosed, piston assemblies of the type disclosed by Kottmann '355 may compromise piston rod-piston assembly strength by eliminating the more conventional cylindrical piston pin configuration which allows the piston pin to be received in mating cylindrical apertures formed in piston head support members 4 and 5. For example, shoulders 20 and 21 of the Kottmann '355 pin 6 are inclined relative to the central axis of the piston assembly and thus tend to impose a radial force on support members 4 and 5 in response to combustion gas pressure. To resist such radial forces, peg-shaped piece 15 and sleeve 16 are provided at opposite ends of bolt 11. The relatively large diameter of peg-shaped piece 15 and sleeve 16 impose requirements for a correspondingly large diameter receiving apparatus in shank 19. These features therefore enlarge the overall size of the piston.